The present invention relates to exhaust gas recirculation effected in internal combustion engines for the purpose of controlling emission of noxious components in exhaust gases from the engine, and, more particularly, to an exhaust gas recirculation control system for controlling the exhaust gas recirculation.
When exhaust gas recirculation is effected in an internal combustion engine, it is important that the amount of exhaust gases recirculated is properly controlled in accordance with the operating conditions of the engine so that the requirement for controlling emission of noxious components is balanced against the requirement for good performance of the engine, i.e. of the vehicle.
As an exhaust gas recirculation control system, there has been proposed a system which employs an electronic control means having a memory means and controls an exhaust gas recirculation control valve directly or indirectly by an electronic signal produced by the electronic control means so as to effect the optimum exhaust gas recirculation in various operating conditions of the engine.
As a type of electronic exhaust gas recirculation control system, it has been proposed in a co-pending Japanese Patent Application No. 56292/77 filed May 18, 1977 to provide a system which depends upon the processes of retaining target values for the intake manifold pressure (absolute) of the engine which provide the optimum exhaust gas recirculation in various operating conditions of the engine determined by the combination of the intake air flow and the rotational speed of the engine, detecting the current values of the intake air flow and the rotational speed of the engine in operation, reading out a particular target value from the retained target values in accordance with the current values of the intake air flow and the rotational speed of the engine, comparing the read-out target value with the current intake manifold pressure of the engine, and controlling the flow of recirculating exhaust gases in a manner to accord the intake manifold pressure of the engine to the read-out target value.
When a vehicle which mounts an engine equipped with the abovementioned exhaust gas control system is operated at high altitude, if the power output condition of the engine is the same as in the operation at sea level, the exhaust gas recirculation is performed at the same rate as in the operation at sea level, because although atmospheric pressure lowers as altitude increases, the target intake manifold pressure retained in the memory means in accordance with the intake air flow and the rotational speed of the engine does not change, and the intake air flow does not change substantially in accordance with altitude if the power output condition of the engine does not change. The target intake manifold pressure is of course set at values which are lower than atmospheric pressure at sea level corresponding to the intake manifold vacuum in various operating conditions. However, when the altitude at which the engine is operated increases, the difference between the actual atmospheric pressure and the target intake manifold pressure decreases, and it may possibly happen that the target intake manifold pressure becomes higher than the actual atmospheric pressure. This means that even when the engine is operating at full load, exhaust gas recirculation is performed. This is not desirable from the point of view of ensuring sufficiently high power even in high altitude operation of the engine.